1. Field of the Invention
This invention relates to thermosetting resinous compositions useful for filling metal cavities and, more particularly, to thermosetting unsaturated polyester resin compositions useful for patching dents in the surfaces of metal articles such as automobile vehicle bodies resulting from collisions.
2. The Prior Art
The use of thermosetting resinous materials as patching compositions for damaged or dented sheet metal is well known. For example, unsaturated polyester resinous compositions have been widely used for this purpose.
The unsaturated polyester resin materials upon which the patching compositions are based are usually solutions of unsaturated polyesters in a polymerizable monomeric solvent such as sytrene, vinyl toluene, acrylates and the like. The unsaturated polyester itself is usually formed by the esterification of glycols with dicarboxylic acids or acid anhydrides. At least a part of the dicarboxylic acid or acid anhydride contains ethylenic unsaturation, e.g. maleic acid, maleic anhydride and the like. Some of the dicarboxylic acid may be free of ethylenic unsaturation, e.g., phthalic acid, adipic acid and the like. The unsaturated polyester resin is dissolved in a polymerizable monomer to prepare the customary resinuous based compositions used in the trade. Usually 10 to 60 parts by weight of styrene and 40 to 90 parts by weight of the unsaturated polyester are mixed to formulate a typical resin binder for the thermosetting patching materials used to repair dented surfaces in damaged automobile bodies.
In preparing thermosetting patching materials using an unsaturated polyester resin binder, the art teaches incorporating an inorganic, mineral powder such as talc, clay or silica sand in the binder to impart impact and structural strength as well as workability to the resin composition.
The art, e.g., U.S. Pat. No. 3,230,184, further teaches that where high strength-to-waist ratios are required, that hollow glass spheres be also incorporated in the unsaturated polyester resin based patching formulation. U.S. Pat. No. 3,873,475 discloses incorporating a mixture of hollow microshperes and an inorganic filler powder in an unsaturated polyester resin to prepare a thermosetting resin formulation suitable as a metal patching material. According to the patent, the filler is incorporated in the polyester resin in major amounts, e.g., the total amount of filler particles incorporated in the polyester resin constitute one half to two thirds the total volume of the patching composition.
The mixture of thermosetting polyester resin binder and inorganic filler has the consistency of a paste. Immediately prior to application to a dented metal surface, the paste is mixed with a cross-linking catalyst such as benzoyl peroxide to accelerate hardening of the patching composition. At the time of application, the filled thermosetting polyester patching composition is partially gelled and thixotropic and can be formed and shaped to the configuration of the dent, and upon cure, the composition becomes a hardened thermoset mass of resin which is confined within the dent.
Metal surfaces of automobile bodies, household appliances and the like having dents, cracks or holes are conventionally repaired with inorganic filler modified thermosetting unsaturated polyester patching compositions in order to eliminate these imperfections. A satisfactory elmination of such imperfections involves subjecting the dents filled with the hardened patching composition to a finishing operation in order to obtain a smooth, uniform surface which blends with the metal surface being repaired so as to be indistinguishable from the rest of the metal surface after a coat of paint has been applied to the surface. Since this subsequent finishing operation is costly and time consuming, the provision of a filler modified unsaturated polyester resin which minimizes the time required for the finishing operation is of great importance. There is, therefore, a continuing interest and desire in the metal repair art, and particularly in the repair of damaged automobile bodies, for a patching composition which after application to a metal cavity or dent hardens rapidly and when hardened is readily susceptible to mechanical finishing procedures, particularly having good workability in the sense of being easily filed and readily sanded and good featherability, i.e., its ability to blend in with adjacent metal surfaces.